Hi Madeleine,
This is such a minefield that the best advice is to consult an authoritative dictionary for those cases you are unsure of. But anyway – here goes.
I have compiled this reply from my old university notes, written before the 1990 orthographic reforms and all literature written before 1990 (in reality probably 2015? If ever) should comply with this schema. After 1990 - who knows; probably a veritable mélange. The reforms, as far as I am aware, are advisory and not mandatory.
(1) ADJECTIVE + NOUN (adjective may precede or follow the noun)
Normally the noun part defines the gender and the adjective agrees with the associated noun and both elements are pluralised:
le cerf-volant (kite) - les cerfs-volants
la basse-cour (farmyard) – les basses-cours)
Exceptions :
le peau-rouge (redskin) - les peaux-rouges (non-PC nowadays !!)
le terre-neuve (dog) – les terres-neuves
le gorge-rouge (robin) – les gorges-rouges
(2) NOUN + NOUN
Normally the gender is defined by the principal noun; generally the first noun and usually both elements are pluralised.
le bateau-mouche (river boat) – les bateaux mouches
la voiture-restaurant (restaurant car) les voitures restaurants
Exceptions (plurals)
les années-lumiere (light years), les soutiens-gorge (bras), les timbres-poste
(postage stamps)
(3) NOUN + PREPOSITION + NOUN
Normally the gender is defined by the initial noun, and only this element is pluralised.
le chef-d’œuvre (masterpiece) - les chefs-d’œuvre
la langue-de-chat (cookie) - les langues-de-chat
Exceptions :
le tête-à-tête, le tête-à-queue (spin)
Plurals : some examples remain invariable e.g.:
des pied-à-terre, des pot-au-feu (stew), des tête-à-tête, des bric-à-brac (junk),
des rez-de-chaussée (ground floor)
(4) PREFIX (or ADVERB) + NOUN
The gender is defined by the noun and only the noun is pluralised.
le demi-tarif (halfprice) – les le demi-tarifs
la demi-bouteille (half bottle) - les la demi-bouteilles
le haut-parleur (loudspeaker – les haut-parleurs
(5) PREPOSITION + NOUN
Normally masculine, this category exhibits considerable variation with regard to gender and pluralisation.
l’après-guerre (m/f !) (post war period) – les après-guerres
l’après-midi (m/f !) (afternoon) – les l’après-midi(s)
le sous-main (blotter) – les sous-main
This can be a difficult category since some apparent PREPOSITIONS are actually ADVERBS and follow the rule outlined above (4), e.g.
la sous-préfecure, la sous-alimentation (malnutrition)
(6)VERB + NOUN
This is the most confused and confusing category (even among the authorities) especially with regard to pluralisation. As expected the verb part usually remains invariable.
In the
plural the noun part takes an ‘s’ if the associated stand-alone noun takes an
‘s’ in the plural
(N.B. There are numerous exceptions and
differences of opinion regarding this “rule”)
le couvre-lit (bedspread) - les couvre-lits, le tire-bouchon (corkscrew) - les
tire-bouchons
un ouvre-boîte (can-opener) – des ouvre-boîtes
But
le porte-monnaie (coin-purse) – les porte-monaie
le porte-musique (music case) - les porte-musique
If the noun
part has a terminal ‘s’ in the singular, it, of course, remains invariable in
the plural
le chauffe-plats (plate-warmer) - les chauffe-plats
le porte-clefs (keyring) - les porte-clefs
Most of the nouns in this category are masculine.
Exception :
la garde-robe (wardrobe) – les garde-robes
The prefix garde when combined with a noun to describe an occupation takes an ‘s’ in the plural but the associated noun part may or may not be pluralised.
le garde-chasse (gamekeeper) – les gardes-chasse(s)
le garde-frontière (border guard) - les gardes-frontières
le/la garde-malade (nurse) – les gardes-malade(s)
In other« garde » type composite nouns « garde » is not pluralised and the associated noun part may or may not be pluralised.
le garde-corps (handrail) – les garde-corps
le garde-feu (fireguard) les garde-feu(x)
A further subcategory is composite nouns where the noun part is non-countable or logically singular. In these cases the entire entity remains invariable in the plural.
le garde-boue (mudguard) - les garde-boue
le pare-brise (windscreen) - les pare-brise
un abat-jour (lampshade) - des abat-jour
le gratte-ciel (skyscraper) – les grate-ciel (although some authorities
tolerate gratte ciels but never gratte-cieux).
1990 reforms recommends the addition of ‘s’ to the second noun element in the plural form in all cases.
Tom